tools/testing/selftests/vm/split_huge_page_test.c: fix application of sizeof to pointer

tools/testing/selftests/vm/split_huge_page_test.c: fix application of sizeof to pointer

The coccinelle check report:

  ./tools/testing/selftests/vm/split_huge_page_test.c:344:36-42:
  ERROR: application of sizeof to pointer

Use "strlen" to fix it.

Link: https://lkml.kernel.org/r/20211012030116.184027-1-davidcomponentone@gmail.com
Signed-off-by: David Yang <davidcomponentone@gmail.com>
Reported-by: Zeal Robot <zealci@zte.com.cn>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>